Many industrial machines are designated to automatically perform the same task over and over again. Such machines generally include some sort of control system to regulate automatic operation of the machine and, in many cases, also include some sort of malfunction detection system to determine when a malfunction has occurred so that the industrial machine should be shut down.
More particularly, such malfunction detection systems typically comprise a plurality of sensors adapted to monitor various aspects of the machine's operation, and signalling means for instructing the industrial machine's control system to shut down the machine when a malfunction is detected. Some sensors are adapted to monitor so-called "cyclic" events in the machine's operation; other sensors are adapted to monitor so-called "non-cyclic" events.
Cyclic events are those events which must occur once in each cycle of the industrial machine's operation. For example, in the case of a power press, examples of cyclic events might be stock feed (i.e., the proper insertion of virgin stock into the press) and parts ejection (i.e., the proper ejection of formed parts from the press). Such events are expected to occur at a preselected time during each cycle of the press (i.e., during a so-called "ready window") and to last for a known duration, and can be monitored by a variety of cyclic sensors which watch for the occurrence of the expected event during the ready window. So long as the sensors successfully detect the occurrence of the expected event during the ready window, the industrial machine is allowed to continue operating; however, if the expected event is not detected during the ready window, the malfunction detection system generates a stop signal to shut down the industrial machine, typically either immediately or at the conclusion of the current machine cycle, depending on the nature of the malfunction. It will also be appreciated that inasmuch as the cyclic events which are being monitored are expected to take place at a known time and last for a known duration during each cycle of the machine, the proper operation of the system's cyclic sensors can be continuously verified by the malfunction detection system, simply by checking to see that a particular cyclic sensor changes state only at the expected time.
"Non-cyclic" events, on the other hand, are those events which do not necessarily occur in each cycle of the industrial machine's operation. For example, in the case of a power press, examples of non-cyclic events might be the occurrence of stock buckling, or the occurrence of a full parts hopper. Such events typically call for shutting down operation of the press, and can be monitored by a variety of sensors which watch for the occurrence of the undesired event. So long as the sensors do not detect the occurrence of the undesired event, the industrial machine is permitted to continue operating; however, if the occurrence of the undesired event is detected, the malfunction detection system generates a stop signal to shut down the industrial machine, either immediately upon the detection of the undesired condition or at the conclusion of the current machine cycle, depending upon the nature of the malfunction. It will also be appreciated that inasmuch as the sensors used to detect non-cyclic events change state only upon the occurrence of undesired events, and since these undesired events typically occur at unpredictable times, the proper operation of the system's non-cyclic sensors cannot be continuously verified by the malfunction detection system in a manner analogous to that of the cyclic sensors as described above.
A third type of event may occur in the course of operating an industrial machine which is neither cyclic nor non-cyclic. Such an event may be referred to as an "intermittent" event, and it is incapable of being properly monitored by either traditional cyclic or traditional non-cyclic sensors. More particularly, intermittent events are those events which need not necessarily occur within each operating cycle of a properly functioning machine, but which must definitely occur at some point within a specified maximum number of machine cycles, as measured from the occurrence of the next preceding one of such intermittent events.
For example, in the case of a power press, an example of an intermittent event might be the proper ejection of waste stock (e.g. "slugs") which can sometimes temporarily collect within the die before falling free. More specifically, in many die designs the waste slugs are intended to sequentially enter a channel which extends between the punching surface of the die and bottom surface of the die, to thereafter fall free of the die. Such slugs may fall free of the die immediately upon being punched from the stock, or several of them may remain within the die (e.g. due to friction with the walls of the channel) before falling free. However, if too many slugs should collect within the die before they are ejected by gravity, perhaps because one of the slugs is bent or otherwise stuck in the die, the slugs may overflow into the adjacent tooling so as to cause a machine jam, or tooling damage, etc.
Thus it will be appreciated that if, for example, the die's channel has a maximum capacity of six slugs, the power press could operate safely for up to six cycles without expelling a slug. Thereafter, however, at least one slug would have to be expelled during the seventh cycle in order to make room at the upper end of the channel for the seventh slug.
If slug expulsion followed a consistent "one slug per cycle" rule, a cyclic sensor could be used to verify that the slugs were being properly ejected from the die and not accumulating in the channel. Unfortunately, however, the slugs do not tend to consistently fall clear of the die with each cycle; instead, they tend to accumulate into multi-slug stacks in the channel until some event (e.g. the accumulating weight of the upper slugs) clears the multi-slug stack as a unit. Using the foregoing example of a channel having a maximum capacity of six slugs, it will be seen that once the accumulated slugs have been cleared, the machine could again operate for up to six cycles prior to the absolute need to once again expel the slugs accumulating in the channel.
Since the slugs do not consistently follow a "one slug per cycle" expulsion rule, ordinary cyclic sensors cannot be used to monitor slug expulsions. As a result, some malfunction detection systems have used non-cyclic sensors, placed within the tooling itself, to monitor a die for the presence of too many accumulated slugs within the channel. Unfortunately, however, this arrangement tends to be an expensive and complex solution to the problem of slug accumulation.